Materials and 3D Designs of Helix Nanostructures for Chirality at Optical Frequencies

The capability to fully control the chiro-optical properties of metamaterials has drawn considerable attention due to developments in metamaterial fabrication techniques and a deeper understanding of the light-matter interaction, enabling a number of applications from integrated photonics to life sciences. From simple geometrical studies on single helix shaped metamaterials, both the design complexity and nanofabrication capability are steadily increasing allowing to extend their properties to the visible regime and beyond. In this work, helix-shaped micro- and nanostructures are reviewed, which have the possibility to modulate their optical response in the linear regime at optical frequencies by spatial arrangement or by constitutive material. An overview of different theories describing the general optical operation of chiral media and a discussion on experimental results obtained by the state-of-the-art helix structures realized by different fabrication techniques is provided. Finally, advanced designs for improved functionality envisaged to match practical applications or to enable new optical multifunctionalities are compared. Even if many applications have been envisaged only theoretically, the continuous effort and progress shown by the research community engaged in this field suggests that helical chiral metamaterials could represent a disruptive breakthrough for advanced optical devices.